As a colorant for various inks and paints, dyes are used in a field requiring higher level of sharpness and printing or coating property and on the other hand, pigments are used for a field requiring water resistance and light resistance, and in particular, the tendency is remarkable in the field of an ink for ink jet recording. A conventional ink jet recording system has been developed from the viewpoint that the maximum problem to be solved is how superior image sharpness and stable spouting property can be imparted to ink. However, the number of prevailed digital cameras has been greatly increased recently because of easy handling and high image quality and the target of ink jet recording extends to the field of silver salt photography. Accordingly, the impartation of light resistance is going to be a problem on par with the sharpness so that color is not faded nevertheless long preservation. Therefore, conventional dyes are going to be rapidly converted to pigments as the colorant of ink, but there remain many problems that must be solved when pigments are utilized.
For example, since a dye is dissolved in dispersion medium in molecular state, it affects little flowability and re-dissolving property dominating the spouting property of the ink even after elapsed time or a concentrated state and can keep high spouting stability. However, a pigment existing in particle state easily causes phenomena such as coagulation, sedimentation and solidification, and such phenomena lower flowability and re-dissolving property and as a result, the spouting stability is deteriorated. In particular, an organic pigment superior in sharpness must be dispersed as fine particles in aqueous medium in the ink jet recording system requiring high sharpness, whereby the area of hydrophobic surface is increased to inevitably cause coagulation.
For dispersing a pigment in an aqueous medium, methods for wetting the hydrophobic surface of the pigment using a dispersant and stably dispersing it in the aqueous medium are usually used. Among them, a method of adsorbing a surfactant or a polymer compound on the surface of a pigment is widely used. However, a strong adsorption force is required for keeping the dispersion state of the pigment and there are little dispersants exhibiting good pigment dispersibility for a long period. Further, since a large quantity of a dispersant is necessary for improving the dispersibility of a pigment by the method, the disengagement speed of the aqueous medium from the system becomes slow and when it is applied to the above-mentioned ink for ink jet recording, there are problems that the drying property of ink is lowered, the pigment is diffused in the inside of a printed article and printing density is also lowered.
Consequently, a method of coating the surface of a pigment with a film forming resin to obtain a resin-coated pigment is proposed as a method of improving the dispersion stability of the pigment and not causing the lowering of drying property and density of a printed article. For example, there are disclosed methods including a method of coating an anionic group-containing resin on the surface of a pigment by a phase transfer emulsification method (JP 10-046075 A and JP 10-140065 A) and a method of coating an anionic group-containing resin on the surface of a pigment by an acid precipitation method (JP 10-140065 A and JP 09-031360 A). In the phase transfer emulsification method, a dispersion in which a pigment is dispersed in an organic solvent solution of an anionic group-containing resin neutralized with a basic compound is prepared, the dispersion is mixed with a large quantity of an aqueous medium, and then, the organic solvent is removed if necessary, to obtain an aqueous pigment dispersion containing a resin-coated pigment. In the acid precipitation method, a pigment is dispersed in an aqueous solution in which an anionic group-containing resin has been dissolved in the presence of a basic compound, and then an acidic compound is added to adjust pH to neutral or acidic condition, thereby coating the surface of the pigment with the anionic group-containing resin to obtain a pigment cake, and then, the resin-coated pigment is dispersed again using a basic compound, to obtain an aqueous pigment dispersion with good dispersibility.
However, when the phase transfer emulsification method is used, the resin is really interacted with only an organic solvent in dispersing a pigment; therefore the expected effect of dispersing the pigment is not obtained. Further, since a large quantity of organic solvent remains in the system, storage stability is lowered and on the other hand, if excessive organic solvent is removed, there is a problem that it is not economically advantageous. When the acid precipitation method is used, the system contains the salt of acid and base which causes the lowering of various performances that are required for the ink for ink jet recording, and there is a problem that adequate washing is required for removing the salt.